More of this Feature • Part 1: Outboard Fuel Filter, Testing Compression, Cold-Weather Epoxy • Part 2: Intercooling, Stalling Stern Drive
Related Resources • Maintenance Q&A Index

What is intercooling, and how is this system maintained? K.L., via e-mail
Because the compressed air leaving a turbocharged engine can reach temperatures of 300°F, this creates two problems. First, hot intake air raises cylinder temperatures, which can damage internal components, especially valves and pistons. Second, hot air is less dense than cool air, so while a turbocharger may be pumping away, the actual volume of air that goes into the cylinder is less than it could be.

Intercooling--sometimes known as aftercooling--cools the air between the time it leaves the turbocharger and the time it enters the valve passages. After air exits the turbocharger, it passes over a series of tubes and fins containing engine coolant or raw water. In most applications a coolant is diverted to the intercooler right after it leaves the raw-water heat exchanger. This is when it is at its lowest temperature, around 180°F, thus providing a substantial temperature reduction in the air. If sea water is used, the temperature is usually between 70 and 80°F, and so the temperature drop is even greater, allowing more fuel to be added and more horsepower to be produced.

Intercooler maintenance usually involves periodic inspection of the raw-water intakes, hoses, clamps, thermostat, raw-water pump and impeller as well as making sure the coolant (if used) is the right mixture. Because raw-water intercooling is more prone to corrosion and clogging, the raw-water intake strainer should be periodically checked for debris, especially eel grass. 

My boat has an older gasoline-powered stern drive that stalls out after high-speed cruising. If I let it cool down, it will restart, but as soon as I begin running at high speed, it stalls again. What can be wrong? F.S., via e-mail
This sounds like vapor lock, which can be caused when the fuel temperature soars due to an overheated engine or running in hot weather. When the gasoline gets too hot in the carburetor bowl, it ceases to flow into the venturis, and the engine basically starves. In extreme cases fuel reaching the fuel pump overheats, and the pump can't handle the resulting vapor. Again, the engine is starved for fuel.

If your engine is overheating, check your cooling system, including the level and mixture of engine coolant (for freshwater-cooled models), thermostat, fan belt, and fuel pump. If your engine is not overheating, your only solutions are insulting the fuel line and changing to a fuel with a special summer "anti-vapor lock" blend.

Need help with a maintenance problem? Write to Maintenance Q & A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: kkreisler@primediasi.com. No phone calls, please.

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